The present invention relates to wireless transmission systems for transmitting data cells, for example asynchronous transfer mode (ATM) cells, wherein the header includes control information (e.g., address information) controlling the destination of the cell.
Conventional asynchronous transfer mode (ATM) systems transmit 53 byte cells comprising a 5 byte header and a 48 byte payload. Typically, the 5 byte header includes 4 bytes of control information (e.g., address information) and an eight bit checksum field called the header error check or code (HEC). The HEC is included as a check that the header information is not corrupted in order to avoid the incorrect forwarding of the cell. This 8 bit checksum field is typically sufficient for this purpose for ATM cells transmitted over fiber optic networks, wherein the medium is relatively reliable and the vast majority of errors are single-bit errors.
However, digital radio transmission systems (e.g., TDMA, CDMA, etc.) for wireless ATM systems are significantly more error prone than fiber optic networks. Furthermore, the errors are not predominantly single bit errors, but rather, are often bursty in nature. Thus, an 8 bit HEC is often insufficient to ensure that the cell (or more particularly, the header portion of the cell) is properly received with sufficient reliability.
Improved forward error correction (FEC) techniques have been proposed for alleviating these problems. However, these techniques still forward cells with uncorrectable errors in their headers in unacceptable proportions for many applications. Note that for a given code length there is a higher probability of detecting an error, using an error detection technique than there is for determining that a detectable but non-correctable error pattern occurred using an FEC technique. Furthermore, some prior art techniques propose FEC be applied to entire cells or blocks of cells, and discarding cells or blocks of cells if the FEC process cannot correct all errors. These techniques are problematic as they discard cells (or group of cells) even if the headers in the discarded cell(s) are correct. In some situations, this is undesirable, as a payload with some errors is better than missing an entire payload for some applications (e.g., voice, video conferencing, etc.).
An object of the present invention is to provide an improved transceiver architecture and method for detecting errors in data cells transmitted by wireless communication. Advantageously, the improved method and architecture for implementing the method more reliably detects header errors than prior art systems. This system thus reduces the likelihood that cells are misdirected.
Advantageously, the inventors have recognised that for a given code length there is a higher probability of detecting an error using an error detection technique than there is for determining that a detectable but non-correctable error pattern occurred using an FEC technique. Furthermore, the inventors have realised that for some applications, as long as the header is correct, there is utility in forwarding a cell with a corrupted payload. Thus, a further object of the invention is to increase the probability of detecting a cell with header errors without increasing the probability of (unintentionally) discarding a received cell which has a valid header compared to prior art systems which rely solely on FEC and/or conventional header error checks.
The improved transceiver architectures and methods can be used in both point-to-point and point-to-multipoint radio units. In either system, the transmitter of a cell reconfigures the cell for transmission such that errors which occur in the header can be detected by the receiving radio unit with greater accuracy than conventional techniques. In particular, a reconfigured cell according to this invention includes enhanced header error detection coding for example, by including an extended header error code (EHEC). The receiving unit retrieves the reconfigured cell from the transmitted radio bit stream and detects header errors using the enhanced header error detection coding.
If no header errors are detected by the receiving unit, various treatments can be applied. Typically, the enhanced header error detection coding is replaced by a conventional header error coding check if no further wireless transmission is required. The system can then simply forward such a cell, relying on higher layer protocols to determine what treatment should be performed on cells with corrupted payloads.
Thus, according to a broad aspect of the invention there is provided a transceiver for receiving and transmitting data cells over a wireless interface, said transceiver comprising: a block processor having a transmit path for reconfiguring cells for transmission over the wireless interface and for receiving transmitted signals and processing received reconfigured cells; wherein said block processor includes: a transmit header processor for assembling a new header for a cell to be transmitted, including: means for extracting HEC from said cell, and means for calculating and inserting an EHEC into said cell header; a receive header processor, including: means for extracting EHEC from a received cell, means for replacing said EHEC with an HEC, and means for detecting errors in said received cell header; and means for discarding a received cell if an error in the header is detected.
In accordance with another aspect of the present invention there is provided a method for treating transmission errors in data cells which include a first data field, a check on said first data field, and at least one additional data field, wherein each cell is transmitted with additional bits for the purpose of allowing a receiver of the cell to determine whether there are uncorrectable errors, said method comprising:
detecting errors on said first data field;
detecting uncorrected errors on said cell;
determining an error is present in said at least one additional data field if an uncorrected error is detected in said cell and no error is detected in said data field; and
executing an appropriate treatment process depending on the nature of the cell and the field in error.
Such a method is particularly advantageous to wireless transmission of data cells wherein said first data field is a header and said at least one additional data field is a payload.
Other aspects of the invention provide for enhanced treatments and optional features. For example, additional Forward Error Correction or error detection coding can be applied, either to cells or blocks of cells. FEC has the advantage of generally improving the radio transmissions. Either way, this additional coding can help determine the type of treatment to be applied. For example, if detectable but uncorrectable errors are detected on a particular cell, but no error is detected in the header, then the system can conclude that there is an error in the payload. This can allow systems to select between various treatments, including:
forwarding the cell,
forwarding the cell with a flag,
discarding the cell, and
producing an indication of the error to a higher layer protocol.
Advantageously, for groups of related cells (hereafter called a message) for which the payload of every cell is necessary, special treatments can be applied to conserve bandwidth by discontinuing the forwarding and/or transmission of subsequent cells of a message once a payload error has been detected in any cell of a message.